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We have a situation where some interceptor valves on one of our 500 megawatt steam turbine generation sets have been sezing following a recent outage. These are a fairly standard design with a hydraulic actualtor oprating the steam valve through a lincage system coupled to a crosshead or guide bush if you like which is fixed to the top of the valve spindle. This bush is approximately 6 inches in dia and about 6.5 inches long. It is here where we believe that the problem is occuring. (it is not the servo as the valve remained jammed following servo removal!)

We are looking into issues surrounding the lubrication of these bushings and mechanical limits and fits. We have been led to believe that the bushes have around 4 thou clearance which is somewhat less than the normal clearance of 10-15 thou.


My question is does anyone have any experience for minimum clearances for grease lubrication for this type of application. Obviously if the grease cannot 'wet' the entire circumference of the bush then problems are going to occur?

regards

Gary

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It would be my belief Gary that 0.004" is awful close to grease for a 6.5" length. I would assume that the grease zerk is in the middle of the bush, so you should have to push the grease only 3 1/4", however, your also pushing it through a 0.002" space, which I would think is too small. That would probably be a good oil clearance, but depending on the viscosity of the grease you're using, that could be to little.
How is it greased? Is it done by hand, or is there an "automatic" greaser hooked to it? If it is a screw on canister type, I guarantee you that it is too small clearance for that. It doesn't take much back pressure to shut one of the best of those things down.

D
RM
Grease will flow within that small space. There won't be much but it doesn't take much either. However it can be sensitive to upset condition. Does the bushing have distribution grooves? Are the temperatures stable? What is the consistency of the grease, and is it the OEM recommendation?

Also with that small clearance, it will be sensitive to edge loading due to misalignment. Has the alignment of the rotating element been confirmed?

David
RM

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